This invention relates to active derivatives of poly(ethylene glycol) and related hydrophilic polymers and to methods for their synthesis for use in modifying the characteristics of surfaces and molecules.
Poly(ethylene glycol) (xe2x80x9cPEGxe2x80x9d) has been studied for use in pharmaceuticals, on artificial implants, and in other applications where biocompatibility is of importance. Various derivatives of poly(ethylene glycol) (xe2x80x9cPEG derivativesxe2x80x9d) have been proposed that have an active moiety for permitting PEG to be attached to pharmaceuticals and implants and to molecules and surfaces generally to modify the physical or chemical characteristics of the molecule or surface.
For example, PEG derivatives have been proposed for coupling PEG to surfaces to control wetting, static buildup, and attachment of other types of molecules to the surface, including proteins or protein residues. More specifically, PEG derivatives have been proposed for attachment to the surfaces of plastic contact lenses to reduce the buildup of proteins and clouding of vision. PEG derivatives have been proposed for attachment to artificial blood vessels to reduce protein buildup and the danger of blockage. PEG derivatives have been proposed for immobilizing proteins on a surface, as in enzymatic catalysis of chemical reactions.
In still further examples, PEG derivatives have been proposed for attachment to molecules, including proteins, for protecting the molecule from chemical attack, to limit adverse side effects of the molecule, or to increase the size of the molecule, thereby potentially to render useful substances that have some medicinal benefit, but are otherwise not useful or are even harmful to a living organism. Small molecules that normally would be excreted through the kidneys are maintained in the blood stream if their size is increased by attaching a biocompatible PEG derivative. Proteins and other substances that create an immune response when injected can be hidden to some degree from the immune system by coupling of a PEG molecule to the protein.
PEG derivatives have also been proposed for affinity partitioning of, for example, enzymes from a cellular mass. In affinity partitioning, the PEG derivative includes a functional group for reversible coupling to an enzyme that is contained within a cellular mass. The PEG and enzyme conjugate is separated from the cellular mass and then the enzyme is separated from the PEG derivative, if desired.
Coupling of PEG derivatives to proteins illustrates some of the problems that have been encountered in attaching PEG to surfaces and molecules. For many surfaces and molecules, the number of sites available for coupling reactions with a PEG derivative is somewhat limited. For example, proteins typically have a limited number and distinct type of reactive sites available for coupling. Even more problematic, some of the reactive sites may be responsible for the protein""s biological activity, as when an enzyme catalyzes certain chemical reactions. A PEG derivative that attached to a sufficient number of such sites could adversely affect the activity of the protein.
Reactive sites that form the loci for attachment of PEG derivatives to proteins are dictated by the protein""s structure. Proteins, including enzymes, are built of various sequences of alpha-amino acids, which have the general structure H2Nxe2x80x94CHRxe2x80x94COOH. The alpha amino moiety (H2Nxe2x80x94) of one amino acid joins to the carboxyl moiety (xe2x80x94COOH) of an adjacent amino acid to form amide linkages, which can be represented as xe2x80x94(NHxe2x80x94CHRxe2x80x94CO)nxe2x80x94, where n can be hundreds or thousands. The fragment represented by R can contain reactive sites for protein biological activity and for attachment of PEG derivatives.
For example, in lysine, which is an amino acid forming part of the backbone of most proteins, an xe2x80x94NH2 moiety is present in the epsilon position as well as in the alpha position. The epsilon xe2x80x94NH2 is free for reaction under conditions of basic pH. Much of the art has been directed to developing PEG derivatives for attachment to the epsilon xe2x80x94NH2 moiety of the lysine fraction of a protein. These PEG derivatives all have in common that the lysine amino acid fraction of the protein typically is inactivated, which can be a drawback where lysine is important to protein activity.
Zalipsky U.S. Pat. No. 5,122,614 discloses that PEG molecules activated with an oxycarbonyl-N-dicarboximide functional group can be attached under aqueous, basic conditions by a urethane linkage to the amine group of a polypeptide. Activated PEG-N-succinimide carbonate is said to form stable, hydrolysis-resistant urethane linkages with amine groups. The amine group is shown to be more reactive at basic pHs of from about 8.0 to 9.5, and reactivity falls off sharply at lower pH. However, hydrolysis of the uncoupled PEG derivative also increases sharply at pH""s of 8.0 to 9.5. Zalipsky avoids the problem of an increase in the rate of reaction of the uncoupled PEG derivative with water by using an excess of PEG derivative to bind to the protein surface. By using an excess, sufficient reactive epsilon amino sites are bound with PEG to modify the protein before the PEG derivative has an opportunity to become hydrolyzed and unreactive.
Zalipsky""s method is adequate for attachment of the lysine fraction of a protein to a PEG derivative at one active site on the PEG derivative. However, if the rate of hydrolysis of the PEG derivative is substantial, then it can be problematic to provide attachment at more than one active site on the PEG molecule, since a simple excess does not slow the rate of hydrolysis.
For example, a linear PEG with active sites at each end will attach to a protein at one end, but, if the rate of hydrolysis is significant, will react with water at the other end to become capped with a relatively nonreactive hydroxyl moiety, represented structurally as xe2x80x94OH, rather than forming a xe2x80x9cdumbbellxe2x80x9d molecular structure with attached proteins or other desirable groups on each end. A similar problem arises if it is desired to couple a molecule to a surface by a PEG linking agent because the PEG is first attached to the surface or couples to the molecule, and the opposite end of the PEG derivative must remain active for a subsequent reaction. If hydrolysis is a problem, then the opposite end typically becomes inactivated.
Also disclosed in Zalipsky U.S. Pat. No. 5,122,614 are several other PEG derivatives from prior patents. PEG-succinoyl-N-hydroxysuccinimide ester is said to form ester linkages that have limited stability in aqueous media, thus indicating an undesirable short half-life for this derivative. PEG-cyanuric chloride is said to exhibit an undesirable toxicity and to be non-specific for reaction with particular functional groups on a protein. The PEG-cyanuric chloride derivative may therefore have undesirable side effects and may reduce protein activity because it attaches to a number of different types of amino acids at various reactive sites. PEG-phenylcarbonate is said to produce toxic hydrophobic phenol residues that have affinity for proteins. PEG activated with carbonyldiimidazole is said to be too slow in reacting with protein functional groups, requiring long reaction times to obtain sufficient modification of the protein.
Still other PEG derivatives have been proposed for attachment to functional groups on amino acids other than the epsilon xe2x80x94NH2 of lysine. Histidine contains a reactive imino moiety, represented structurally as xe2x80x94N(H)xe2x80x94, but many derivatives that react with epsilon xe2x80x94NH2 also react with xe2x80x94N(H)xe2x80x94. Cysteine contains a reactive thiol moiety, represented structurally as xe2x80x94SH, but the PEG derivative maleimide that is reactive with this moiety is subject to hydrolysis.
As can be seen from the small sampling above, considerable effort has gone into developing various PEG derivatives for attachment to, in particular, the xe2x80x94NH2 moiety on the lysine amino acid fraction of various proteins. Many of these derivatives have proven problematic in their synthesis and use. Some form unstable linkages with the protein that are subject to hydrolysis and therefore do not last very long in aqueous environments, such as in the blood stream. Some form more stable linkages, but are subject to hydrolysis before the linkage is formed, which means that the reactive group on the PEG derivative may be inactivated before the protein can be attached. Some are somewhat toxic and are therefore less suitable for use in vivo. Some are too slow to react to be practically useful. Some result in a loss of protein activity by attaching to sites responsible for the protein""s activity. Some are not specific in the sites to which they will attach, which can also result in a loss of desirable activity and in a lack of reproducibility of results.
The invention provides water soluble and hydrolytically stable derivatives of poly(ethylene glycol) (xe2x80x9cPEGxe2x80x9d) polymers and related hydrophilic polymers having one or more active sulfone moieties. These polymer derivatives with active sulfone moieties are highly selective for coupling with thiol moieties instead of amino moieties on molecules and on surfaces, especially at pHs of about 9 or less. The sulfone moiety, the linkage between the polymer and the sulfone moiety, and the linkage between the thiol moiety and the sulfone moiety are not generally reversible in reducing environments and are stable against hydrolysis for extended periods in aqueous environments at pHs of about 11 or less. Consequently, the physical and chemical characteristics of a wide variety of substances can be modified under demanding aqueous conditions with the active sulfone polymer derivatives.
For example, conditions for modification of biologically active substances can be optimized to preserve a high degree of biological activity. Pharmaceuticals from aspirin to penicillin can be usefully modified by attachment of active sulfone polymer derivatives if these pharmaceuticals are modified to contain thiol moieties. Large proteins containing cysteine units, which have active thiol moieties, can also be usefully modified. Techniques of recombinant DNA technology (xe2x80x9cgenetic engineeringxe2x80x9d) can be used to introduce cysteine groups into desired places in a protein. These cysteines can be coupled to active sulfone polymer derivatives to provide hydrolytically stable linkages on a variety of proteins that do not normally contain cysteine units.
Specific sulfone moieties for the activated polymers of the invention are those having at least two carbon atoms joined to the sulfone group xe2x80x94SO2xe2x80x94 with a reactive site for thiol specific coupling reactions on the second carbon from the sulfone group.
More specifically, the active sulfone moieties comprise vinyl sulfone, the active ethyl sulfones, including the haloethyl sulfones, and the thiol-specific active derivatives of these sulfones. The vinyl sulfone moiety can be represented structurally as xe2x80x94SO2xe2x80x94CHxe2x95x90CH2; the active ethyl sulfone moiety can be represented structurally as xe2x80x94SO2xe2x80x94CH2xe2x80x94CH2Z, where Z can be halogen or some other leaving group capable of substitution by thiol to form the sulfone and thiol linkage xe2x80x94SO2xe2x80x94CH2xe2x80x94CH2xe2x80x94Sxe2x80x94W, where W represents a biologically active molecule, a surface, or some other substance. The derivatives of the vinyl and ethyl sulfones can include other substituents, so long as the water solubility and the thiol-specific reactivity of the reactive site on the second carbon are maintained.
The invention includes hydrolytically stable conjugates of substances having thiol moieties with polymer derivatives having active sulfone moieties. For example, a water soluble sulfone-activated PEG polymer can be coupled to a biologically active molecule at a reactive thiol site. The linkage by which the PEG and the biologically active molecule are coupled includes a sulfone moiety coupled to a thiol moiety and has the structure PEG-SO2xe2x80x94CH2xe2x80x94CH2xe2x80x94Sxe2x80x94W, where W represents the biologically active molecule, whether the sulfone moiety prior to coupling of the PEG was vinyl sulfone or an active ethyl sulfone.
The invention also includes biomaterials comprising a surface having one or more reactive thiol sites and one or more of the water soluble sulfone-activated polymers of the invention coupled to the surface by a sulfone and thiol linkage. Biomaterials and other substances can also be coupled to the sulfone activated polymer derivatives through a linkage other than the sulfone and thiol linkage, such as a conventional amino linkage, to leave a more hydrolytically stable activating group, the sulfone moiety, available for subsequent reactions.
The invention includes a method of synthesizing the activated polymers of the invention. A sulfur containing moiety is bonded directly to a carbon atom of the polymer and then converted to the active sulfone moiety. Alternatively, the sulfone moiety can be prepared by attaching a linking agent that has the sulfone moiety at one terminus to a conventional activated polymer so that the resulting polymer has the sulfone moiety at its terminus.
More specifically, a water soluble polymer having at least one active hydroxyl moiety undergoes a reaction to produce a substituted polymer having a more reactive moiety thereon. The resulting substituted polymer undergoes a reaction to substitute for the more reactive moiety a sulfur-containing moiety having at least two carbon atoms where the sulfur in the sulfur-containing moiety is bonded directly to a carbon atom of the polymer. The sulfur-containing moiety then undergoes reactions to oxidize sulfur, xe2x80x94Sxe2x80x94, to sulfone, xe2x80x94SO2xe2x80x94, and to provide a sufficiently reactive site on the second carbon atom of the sulfone containing moiety for formation of linkages with thiol containing moieties.
Still more specifically, the method of synthesizing the activated polymers of the invention comprises reacting poly(ethylene glycol) with a hydroxyl activating compound to form an ester or with a halogen containing derivative to form a halogen substituted PEG. The resulting activated PEG is then reacted with mercaptoethanol to substitute the mercaptoethanol radical for the ester moiety or the halide. The sulfur in the mercaptoethanol moiety is oxidized to sulfone. The ethanol sulfone is activated by either activating the hydroxyl moiety or substituting the hydroxyl moiety with a more active moiety such as halogen. The active ethyl sulfone of PEG can then be converted to vinyl sulfone, if desired, by cleaving the activated hydroxyl or other active moiety and introducing the carbon-carbon double bond adjacent the sulfone group xe2x80x94SO2xe2x80x94.
The invention also includes a method for preparing a conjugate of a substance with a polymer derivative that has an active sulfone moiety. The method includes the step of forming a linkage between the polymer derivative and the substance, which linkage can be between the sulfone moiety and a thiol moiety.
Thus the invention provides activated polymers that are specific in reactivity, stable in water, stable in reducing environments, and that form more stable linkages with surfaces and molecules, including biologically active molecules, than previously has been achieved. The activated polymer can be used to modify the characteristics of surfaces and molecules where biocompatibility is of importance. Because the activated polymer is stable under aqueous conditions and forms stable linkages with thiol moieties, the most favorable reaction conditions can be selected for preserving activity of biologically active substances and for optimizing the rate of reaction for polymer coupling.
The synthetic route used to prepare active sulfones of poly(ethylene glycol) and related polymers comprises at least four steps in which sulfur is bound to a polymer molecule and then converted through a series of reactions to an active sulfone functional group. The starting PEG polymer molecule has at least one hydroxyl moiety, xe2x80x94OH, that is available to participate in chemical reactions and is considered to be an xe2x80x9cactivexe2x80x9d hydroxyl moiety. The PEG molecule can have multiple active hydroxyl moieties available for chemical reaction, as is explained below. These active hydroxyl moieties are in fact relatively nonreactive, and the first step in the synthesis is to prepare a PEG having a more reactive moiety.
A more reactive moiety typically will be created by one of two routes, hydroxyl activation or substitution. Other methods are available as should be apparent to the skilled artisan, but hydroxyl activation and substitution are the two most often used. In hydroxyl activation, the hydrogen atom xe2x80x94H on the hydroxyl moiety xe2x80x94OH is replaced with a more active group. Typically, an acid or an acid derivative such as an acid halide is reacted with the PEG to form a reactive ester in which the PEG and the acid moiety are linked through the ester linkage. The acid moiety generally is more reactive than the hydroxyl moiety. Typical esters are the sulfonate, carboxylate, and phosphate esters.
Sulfonyl acid halides that are suitable for use in practicing the invention include methanesulfonyl chloride and p-toluenesulfonyl chloride. Methanesulfonyl chloride is represented structurally as CH3SO2Cl and is also known as mesyl chloride. Methanesulfonyl esters are sometimes referred to as mesylates. Para-toluenesulfonyl chloride is represented structurally as H3CC6H4SO2Cl and is also known as tosyl chloride. Toluenesulfonyl esters are sometimes referred to as tosylates.
In a substitution reaction, the entire xe2x80x94OH group on the PEG is substituted by a more reactive moiety, typically a halide. For example, thionyl chloride, represented structurally as SOCl2, can be reacted with PEG to form a more reactive chlorine substituted PEG. Substitution of the hydroxyl moiety by another moiety is sometimes referred to in the art as hydroxyl activation. The term xe2x80x9chydroxyl activationxe2x80x9d should be interpreted herein to mean substitution as well as esterification and other methods of hydroxyl activation.
The terms xe2x80x9cgroup,xe2x80x9d xe2x80x9cfunctional group,xe2x80x9d xe2x80x9cmoiety,xe2x80x9d xe2x80x9cactive moiety,xe2x80x9d xe2x80x9creactive site,xe2x80x9d and xe2x80x9cradicalxe2x80x9d are somewhat synonymous in the chemical arts and are used in the art and herein to refer to distinct, definable portions or units of a molecule and to units that perform some function or activity and are reactive with other molecules or portions of molecules. In this sense a protein or a protein residue can be considered a molecule or as a functional group or moiety when coupled to a polymer.
The term xe2x80x9cPEGxe2x80x9d is used in the art and herein to describe any of several condensation polymers of ethylene glycol having the general formula represented by the structure H(OCH2CH2)nOH, which can also be represented as HOxe2x80x94CH2CH2xe2x80x94(OCH2CH2)nxe2x80x94OH. PEG is also known as polyoxyethylene, polyethylene oxide, polyglycol, and polyether glycol. PEG can be prepared as copolymers of ethylene oxide and many other monomers.
Poly(ethylene glycol) is used in biological applications because it has properties that are highly desirable and is generally approved for biological or biotechnical applications. PEG typically is clear, colorless, odorless, soluble in water, stable to heat, inert to many chemical agents, does not hydrolyze or deteriorate, and is nontoxic. Poly(ethylene glycol) is considered to be biocompatible, which is to say that PEG is capable of coexistence with living tissues or organisms without causing harm. More specifically, PEG is not immunogenic, which is to say that PEG does not tend to produce an immune response in the body. When attached to a moiety having some desirable function in the body, the PEG tends to mask the moiety and can reduce or eliminate any immune response so that an organism can tolerate the presence of the moiety. Accordingly, the sulfone-activated PEGs of the invention should be substantially non-toxic and should not tend substantially to produce an immune response or cause clotting or other undesirable effects.
The second step in the synthesis is to link sulfur directly to a carbon atom in the polymer and in a form that can be converted to an ethyl sulfone or ethyl sulfone derivative having similar reactive properties. xe2x80x9cEthylxe2x80x9d refers to a moiety having an identifiable group of two carbon atoms joined together. The active sulfone PEG derivative requires that the second carbon atom in the chain away from the sulfone group provide a reactive site for linkages of thiol moieties with the sulfone. This result can be achieved by reacting the active moiety produced in the first step mentioned above, which typically will be the ester or halide substituted PEG, in a substitution reaction with an alcohol that also contains a reactive thiol moiety attached to an ethyl group, a thioethanol moiety. The thiol moiety is oxidized to sulfone and the second carbon away from the sulfone on the ethyl group is converted to a reactive site.
Compounds containing thiol moieties, xe2x80x94SH, are organic compounds that resemble alcohols, which contain the hydroxyl moiety xe2x80x94OH, except that in thiols, the oxygen of at least one hydroxyl moiety is replaced by sulfur. The activating moiety on the PEG derivative from the first reaction, which typically is either halide or the acid moiety of an ester, is cleaved from the polymer and is replaced by the alcohol radical of the thioethanol compound. The sulfur in the thiol moiety of the alcohol is linked directly to a carbon on the polymer.
The alcohol should be one that provides a thioethanol moiety for attachment directly to the carbon of the polymer chain, or that can easily be converted to a thioethanol moiety or substituted moiety of similar reactive properties. An example of such an alcohol is mercaptoethanol, which is represented structurally as HSCH2CH2OH and is sometimes also called thioethanol.
In the third step of the synthesis, an oxidizing agent is used to convert the sulfur that is attached to the carbon to the sulfone group, xe2x80x94SO2. There are many such oxidizing agents, including hydrogen peroxide and sodium perborate. A catalyst, such as tungstic acid, can be useful. However, the sulfone that is formed is not in a form active for thiol-selective reactions and it is necessary to remove the relatively unreactive hydroxyl moiety of the alcohol that was added in the substitution reaction of the second step.
In the fourth step, the hydroxyl moiety of the alcohol that was added in the second step is converted to a more reactive form, either through activation of the hydroxyl group or through substitution of the hydroxyl group with a more reactive group, similar to the first step in the reaction sequence. Substitution typically is with halide to form a haloethyl sulfone or a derivative thereof having a reactive site on the second carbon removed from the sulfone moiety. Typically, the second carbon on the ethyl group will be activated by a chloride or bromide halogen. Hydroxyl activation should provide a site of similar reactivity, such as the sulfonate ester. Suitable reactants are the acids, acid halides, and others previously mentioned in connection with the first step in the reaction, especially thionyl chloride for substitution of the hydroxyl group with the chlorine atom.
The resulting polymeric activated ethyl sulfone is stable, isolatable, and suitable for thiol-selective coupling reactions. As shown in the examples, PEG chloroethyl sulfone is stable in water at a pH of about 7 or less, but nevertheless can be used to advantage for thiol-selective coupling reactions at conditions of basic pH up to at least about pH 9.
In the thiol coupling reaction, it is possible that the thiol moiety displaces chloride, as in the following reaction:
PEG-SO2xe2x80x94CH2xe2x80x94CH2xe2x80x94Cl+Wxe2x80x94Sxe2x80x94Hxe2x86x92PEG-SO2xe2x80x94CH2xe2x80x94CH2xe2x80x94Sxe2x80x94W,
where W represents the moiety to which the thiol moiety SH is linked and can be a biologically active molecule, a surface, or some other substance. However, and while not wishing to be bound by theory, it is believed, based on the observable reaction kinetics as shown in Example 3, that the chloroethyl and other activated ethyl sulfones and reactive derivatives are converted to PEG vinyl sulfone, and that it is the PEG vinyl sulfone or derivative thereof that is actually linked to the thiol moiety. Nevertheless, the resulting sulfone and thiol linkage is not distinguishable, whether from active PEG ethyl sulfone or from PEG vinyl sulfone, and so the active ethyl sulfone can be used at pHs above 7 for linking to thiol groups.
PEG vinyl sulfone is also stable and isolatable and can form thiol-selective, hydrolytically stable linkages, typically in much less time than the haloethyl sulfone or other activated ethyl sulfone, as explained further below.
In a fifth step that can be added to the synthesis, the activated ethyl sulfone is reacted with any of a variety of bases, such as sodium hydroxide or triethylamine, to form PEG vinyl sulfone or one of its active derivatives for use in thiol-selective coupling reactions.
As shown in the examples below, especially Example 3, PEG vinyl sulfone reacts quickly with thiol moieties and is stable against hydrolysis in water of pH less than about 11 for at least several days. The reaction can be represented as follows:
PEG-SO2xe2x80x94CHxe2x95x90CH2+Wxe2x80x94Sxe2x80x94Hxe2x86x92PEG-SO2xe2x80x94CH2xe2x80x94CH2xe2x80x94Sxe2x80x94W.
The thiol moiety is said to add xe2x80x9cacross the double bond.xe2x80x9d The Wxe2x80x94S moiety adds to the terminal CH2 of the double bond, which is the second carbon from the sulfone group SO2. The hydrogen H adds to the CH of the double bond. However, at a pH of above about 9, selectivity of the sulfone moiety for thiol is diminished and the sulfone moiety becomes somewhat more reactive with amino groups.
Alternatively to the above synthesis, the sulfone-activated PEG derivatives can be prepared by attaching a linking agent having a sulfone moiety to a PEG activated with a different functional group. For example, an amino activated PEG, PEG-NH2, is reacted under favorable conditions of pH of about 9 or less with a small molecule that has a succinimidyl active ester moiety NHSxe2x80x94O2Cxe2x80x94 at one terminus and a sulfone moiety, vinyl sulfone xe2x80x94SO2xe2x80x94CHxe2x95x90CH2, at the other terminus. The amino activated PEG forms a stable linkage with the succinimidyl ester. The resulting PEG is activated with the vinyl sulfone moiety at the terminus and is hydrolytically stable. The reaction and the resulting vinyl sulfone activated PEG are represented structurally as follows:
PEG-NH2+NHSxe2x80x94O2Cxe2x80x94CH2xe2x80x94CH2xe2x80x94SO2xe2x80x94CHxe2x95x90CH2xe2x86x92PEG-NHxe2x80x94OCxe2x80x94CH2xe2x80x94CH2xe2x80x94SO2xe2x80x94CHxe2x95x90CH2.
A similar activated PEG could be achieved by reacting an amine-activated PEG such as succinimidyl active ester PEG, PEG-CO2xe2x80x94NHS, with a small molecule that has an amine moiety at one terminus and a vinyl sulfone moiety at the other terminus. The succinimidyl ester forms a stable linkage with the amine moiety as follows:
PEG-CO2xe2x80x94NHS+NH2xe2x80x94CH2xe2x80x94CH2xe2x80x94SO2xe2x80x94CHxe2x95x90CH2xe2x86x92PEG-COxe2x80x94NHxe2x80x94CH2xe2x80x94CH2xe2x80x94SO2xe2x80x94CHxe2x95x90CH2.
The active PEG sulfones of the invention can be of any molecular weight and can be linear or branched with hundreds of arms. The PEG can be substituted or unsubstituted so long as at least one reactive site is available for substitution with the sulfone moieties. PEG typically has average molecular weights of from 200 to 100,000 and its biological properties can vary with molecular weight and depending on the degree of branching and substitution, so not all of these derivatives may be useful for biological or biotechnical applications. For most biological and biotechnical applications, substantially linear, straight-chain PEG vinyl sulfone or bis vinyl sulfone or activated ethyl sulfone will be used, substantially unsubstituted except for the vinyl sulfone or ethyl sulfone moieties and, where desired, other additional functional groups. For many biological and biotechnical applications, the substituents would typically be unreactive groups such as hydrogen Hxe2x80x94 and methyl CH3xe2x80x94(xe2x80x9cm-PEGxe2x80x9d)).
The PEG can have more than one vinyl sulfone or precursor moiety attached or the PEG can be capped on one end with a relatively nonreactive moiety such as the methyl radical, xe2x80x94CH3. The capped form can be useful, for example, if it is desirable simply to attach the polymer chains at various thiol sites along a protein chain. Attachment of PEG molecules to a biologically active molecule such as a protein or other pharmaceutical or to a surface is sometimes referred to as xe2x80x9cPEGylation.xe2x80x9d
A linear PEG with active hydroxyls at each end can be activated at each end with vinyl sulfone or its precursor or derivatives of similar reactivity to become bifunctional. The bifunctional structure, PEG bis vinyl sulfone, for example, is sometimes referred to as a dumbbell structure and can be used, for example, as a linker or spacer to attach a biologically active molecule to a surface or to attach more than one such biologically active molecule to the PEG molecule. The stability of the sulfone moiety against hydrolysis makes it particularly useful for bifunctional or heterobifunctional applications.
Another application for PEG vinyl sulfone and its precursor is dendritic activated PEG in which multiple arms of PEG are attached to a central core structure. Dendritic PEG structures can be highly branched and are commonly known as xe2x80x9cstarxe2x80x9d molecules. Star molecules are generally described in Merrill U.S. Pat. No. 5,171,264, the contents of which are incorporated herein by reference. The sulfone moieties can be used to provide an active, functional group on the end of the PEG chain extending from the core and as a linker for joining a functional group to the star molecule arms.
PEG vinyl sulfone and its precursors and derivatives can be used for attachment directly to surfaces and molecules having a thiol moiety. However, more typically a heterobifunctional PEG derivative having a sulfone moiety on one terminus and a different functional moiety on the opposite terminus group will be attached by the different moiety to a surface or molecule. When substituted with one of the other active moieties, the heterobifunctional PEG dumbbell structure can be used, for example, to carry a protein or other biologically active molecule by sulfone linkages on one end and by another linkage on the other end, such as an amine linkage, to produce a molecule having two different activities. A heterobifunctional PEG having a sulfone moiety on one end and an amine specific moiety on the other end could be attached to both cysteine and lysine fractions of proteins. A stable amine linkage can be achieved and then the hydrolytically stable unreacted sulfone moiety is available for subsequent thiol-specific reactions as desired.
Other active groups for heterobifunctional sulfone-activated PEGs can be selected from among a wide variety of compounds. For biological and biotechnical applications, the substituents would typically be selected from reactive moieties typically used in PEG chemistry to activate PEG such as the aldehydes, trifluoroethylsulfonate, which is also sometimes called tresylate, n-hydroxylsuccinimide ester, cyanuric chloride, cyanuric fluoride, acyl azide, succinate, the p-diazo benzyl group, the 3-(p-diazophenyloxy)-2-hydroxy propyloxy group, and others.
Examples of active moieties other than sulfone are shown in Davis et al. U.S. Pat. No. 4,179,337; Lee et al. U.S. Pat. Nos. 4,296,097 and 4,430,260; Iwasaki et al. 4,670,417; Katre et al. U.S. Pat. Nos. 4,766,106; 4,917,888; and 4,931,544; Nakagawa et al. U.S. Pat. No. 4,791,192; Nitecki et al. U.S. Pat. Nos. 4,902,502 and 5,089,261; Saifer U.S. Pat. No. 5,080,891; Zalipsky U.S. Pat. No. 5,122,614; Shadle et al. U.S. Pat. No. 5,153,265; Rhee et al. U.S. Pat. No. 5,162,430; European Patent Application Publication No. 0 247 860; and PCT International Application Nos. U.S. Ser. No. 86/01252; GB89/01261; GB89/01262; GB89/01263; U.S. Ser. No. 90/03252; U.S. Ser. No. 90/06843; U.S. Ser. No. 91/06103; U.S. Ser. No. 92/00432; and U.S. Ser. No. 92/02047, the contents of which are incorporated herein by reference.
It should be apparent to the skilled artisan that the dumbbell structures discussed above could be used to carry a wide variety of substituents and combinations of substituents. Pharmaceuticals such as aspirin, vitamins, penicillin, and others too numerous to mention; polypeptides or proteins and protein fragments of various functionalities and molecular weights; cells of various types; surfaces for biomaterials, almost any substance could be modified. As used herein, the term xe2x80x9cproteinxe2x80x9d should be understood to include peptides and polypeptides, which are polymers of amino acids. The term xe2x80x9cbiomaterialxe2x80x9d means a material, typically synthetic and sometimes made of plastic, that is suitable for implanting in a living body to repair damaged or diseased parts. An example of a biomaterial is artificial blood vessels.
One straight chain PEG derivative of the invention for biological and biotechnical (applications has the basic structure Rxe2x80x94CH2CH2xe2x80x94(OCH2CH2)nxe2x80x94Y. The PEG monomer OCH2CH2 preferably is substantially unsubstituted and unbranched along the polymer backbone. The subscript xe2x80x9cnxe2x80x9d can equal from about 5 to 3,000. A more typical range is from about 5 to 2,200, which corresponds to a molecular weight of from about 220 to 100,000. Still more typical is a range of from about 34 to 1,100, which corresponds to a molecular weight range of from about 1,500 to 50,000. Most applications will be accomplished with molecular weights in the neighborhood of 2,000 to 5,000, which corresponds to a value of n of from about 45 to 110.
In the above structure, Y represents xe2x80x94SO2xe2x80x94CHxe2x95x90CH2 or xe2x80x94SO2xe2x80x94CH2xe2x80x94CH2xe2x80x94X where X is a halogen. R represents a group that may be the same or different from Y. R can be HOxe2x80x94, H3COxe2x80x94, CH2xe2x95x90CHxe2x80x94SO2xe2x80x94, Clxe2x80x94CH2xe2x80x94CH2xe2x80x94SO2xe2x80x94, or a polymer activating group other than CH2xe2x95x90CHxe2x80x94SO2, Clxe2x80x94CH2xe2x80x94CH2xe2x80x94SO2xe2x80x94, such as is referred to with respect to the above patents and published patent applications.
The active polymer derivatives are water soluble and hydrolytically stable and produce water soluble and hydrolytically stable linkages with thiol groups. The derivatives are considered infinitely soluble in water or as approaching infinite solubility and can enable otherwise insoluble molecules to pass into solution when conjugated with the derivative.
Hydrolytic stability of the derivatives means that the linkage between the polymer and the sulfone moiety is stable in water and that the vinyl sulfone moiety does not react with water at a pH of less than about 11 for an extended period of time of at least several days, and potentially indefinitely, as shown in Example 3 below. The activated ethyl sulfone can be converted to the vinyl sulfone at conditions of basic pH, with the same resulting stability. Hydrolytic stability of the thiol linkage means that conjugates of the activated polymer and a substance having a thiol moiety are stable at the sulfone-thiol linkage for an extended period of time in aqueous environments at a pH of below about 11. Most proteins could be expected to lose their activity at a caustic pH of 11 or higher, so it should be apparent to the skilled artisan that many applications for the active sulfone PEG derivatives will be at pHs of less than 11, regardless of the stability of the sulfone moiety at higher pH.
To be useful for modification of proteins and other substances, it is only necessary that the sulfone be stable for a period of time sufficient to permit the sulfone to react with a reactive thiol moiety on the protein or other substance. The rate of reaction of the sulfone moiety with thiol can vary with pH, as shown in Example 2 below, from about 2 minutes to 30 minutes, which is much faster than the rate of hydrolysis, if any. Vinyl sulfone could be expected to react with thiol over a much broader range of reaction times since it is stable for long periods of time. Also, as shown in Example 3 below, at conditions of basic pH chloroethyl sulfone is not hydrolyzed, but is converted to vinyl sulfone, which remains stable for several days and is even more reactive toward thiol groups. Accordingly, for the purpose of modifying the characteristics of substances, the active ethyl sulfones can also be considered to be hydrolytically stable for an extended period of time over a broad pH range.
Other water soluble polymers than PEG are believed to be suitable for similar modification and activation with an active sulfone moiety. These other polymers include poly(vinyl alcohol) (xe2x80x9cPVAxe2x80x9d); other poly(alkylene oxides) such as poly(propylene glycol) (xe2x80x9cPPGxe2x80x9d) and the like; and poly(oxyethylated polyols) such as poly(oxyethylated glycerol), poly(oxyethylated sorbitol), and poly(oxyethylated glucose), and the like. The polymers can be homopolymers or random or block copolymers and terpolymers based on the monomers of the above polymers, straight chain or branched, or substituted or unsubstituted similar to PEG, but having at least one active site available for reaction to form the sulfone moiety.
The following Example 1 shows the synthesis, isolation, and characterization of poly(ethylene glycol) chloroethyl sulfone followed by the preparation of poly(ethylene glycol) vinyl sulfone from the chloroethyl sulfone. Preparation of other polymeric sulfones having a reactive site on the second carbon from the sulfone group is similar and the steps for doing so should be apparent to the skilled artisan based on Example 1 below and the polymers listed above.